Spark plug

Abstract

A spark plug includes a metal shell, an insulator, a center electrode, a ground electrode, and a terminal fixed to the insulator. The dimensional parameters in the structure of the spark plug, such as a maximum diameter A of the terminal and a diameter B of the insulator satisfy a dimensional relationship defined through experimental investigation. The structure facilitates an attachment of a plug cap to the spark plug.

1. A spark plug comprising:
 a metal shell having a first end and a second end, the metal shell also having a threaded portion on an outer periphery thereof; 
 an insulator having a first end and a second end, the insulator also having a bore formed therein, the insulator being fixed in the metal shell so that the first end of the insulator protrudes from the first end of the metal shell and the second end of the insulator protrudes from the second end of the metal shell; 
 a center electrode secured in the bore of the insulator, the center electrode having a first end protruding from the first end of the insulator and a second end; 
 a ground electrode joined to the first end of the metal shell so that the ground electrode is opposed to the center electrode through a spark gap; 
 a terminal having a first end and a second end, the terminal being partially inserted into the bore and fixed to the insulator so that the first end of the terminal electrically connects to the second end of the center electrode and the second end of the terminal protrudes from the second end of the insulator; and 
 wherein an absolute value of a diameter difference represented by |B−A| is equal to or less than 0.6 mm, where 
 A is a maximum diameter of a protruding portion of the terminal, protruded from the second end of the insulator, and 
 B is a diameter of a protruding portion of the insulator, protruded from the second end of the metal shell. 
 
   
   
     2. The spark plug according to  claim 1 , wherein
 a dimensional relationship B>A is satisfied, and 
 a diameter difference represented by (B−A) is equal to or less than 0.6 mm. 
 
   
   
     3. The spark plug according to  claim 1 , wherein
 an absolute value of a diameter difference represented by |B−A| is equal to or less than 0.4 mm. 
 
   
   
     4. The spark plug according to  claim 1 , wherein
 a diameter C of an inserting portion of the terminal inserted into the insulator, is equal to or less than 3.5 mm. 
 
   
   
     5. The spark plug according to  claim 1 , wherein
 a length L of a protruding portion of the insulator protruded from the metal shell is equal to or greater than 15 mm. 
 
   
   
     6. The spark plug according to  claim 5 , wherein
 the length L is equal to or less than 30 mm. 
 
   
   
     7. The spark plug according to  claim 1 , wherein
 a first noble metal chip joined to the first end of the center electrode, the first noble metal chip of the center electrode has a cross-sectional area in a range of 0.07 to 0.40 mm 2 . 
 
   
   
     8. The spark plug according to  claim 7 , wherein
 the first noble metal chip of the center electrode is made of an Ir-based alloy including Ir in an amount of greater than 50 weight percent and at least one additive, the Ir-based alloy having a melting point of greater than 2000 degrees Celsius. 
 
   
   
     9. The spark plug according to  claim 8 , wherein
 the first noble metal chip of the center electrode including at least one additive is selected from Pt, Rh, Ni, W, Pd, Ru, Re, Al, Al 2 0 3 , Y, Y 2 0 3 . 
 
   
   
     10. The spark plug according to  claim 1 , wherein
 a second noble metal chip is joined to the ground electrode so that the second noble metal chip of the ground electrode is opposed to the first end of the center electrode, 
 the second noble metal chip of the ground electrode has a cross-sectional area in a range of 0.12 to 0.80 mm 2 ′ 
 the second noble metal chip of the ground electrode has a protrusion length from the ground electrode in a range of 0.3 to 1.5 mm, and 
 a spark gap between the center electrode and the ground electrode is in a range of 0.4 to 0.8 mm. 
 
   
   
     11. The spark plug according to  claim 10 , wherein
 the second noble metal chip of the ground electrode is made of an Pt-based alloy including Pt in an amount of greater than 50 weight percent and at least one additive having a melting point of greater than 1500 degrees Celsius. 
 
   
   
     12. The spark plug according to  claim 11 , wherein
 the second noble metal chip of the ground electrode including at least one additive is selected from Ir, Rh, Ni, W, Pd, Ru, Re. 
 
   
   
     13. The spark plug according to  claim 1 , wherein
 the second end of the terminal is capable of attaching an ignition coil.